Integumentary — Hair

The Anatomy & Physiology of Human Hair:

For anatomy related to hair see the labeled images here

Hair is found almost everywhere on the human body except on our eyelids, palms of our hands, bottoms of our feet, and parts of our genitals.

03 hair Fedor_Jeftichew_portrait 02

Fedor Jeftichew worked in side shows and circuses as “Jo Jo the dog faced boy.”  He had hypertrichosis

Vellus hair:  Over most of our body hair does not grow very long, but you can still see little hairs even on your forehead and earlobes if you look close.  These very short hairs are called vellus hair.  There is an interesting disorder called hypertrichosis in which even the vellus hair grows long.  This allows us to better see that hair grows almost everywhere on the human body.

Terminal hair:  This is the type of hair that grows on the top of the head, the eyebrows, and in the adult axillary and groin regions.  Body and facial terminal hair is also called androgenic hair because it is responsive to the hormone testosterone that signals it to grow longer starting a puberty.

Lanugo:  This is a type of very fine hair seen only on the fetus during the last few months of development.

Functions of hair: The major functions of hair include sensory function, protection from UV radiation, reduce friction, heat retention, gender identification, and communication.

  • Sensory function:  Each hair grows out of an oblique tube called a hair follicle.  At the base of each hair follicle is a hair plexus, a sensitive collection of nerve fibers that detect when the hair moves.  This is part of our sense of touch, it is part of how we might feel a bug crawling on us, a loved one’s soft touch, or wind blowing over our skin.
  • Protection from UV radiation:  Any hair growing in great enough amounts and with adequate pigmentation (melanin pigments) is also effective at blocking UV radiation.  This is especially true for the top of your head.
  • Reduce Friction:  In adults any area where skin rubs against itself during normal walking or running has adequate hair to decrease friction between the rubbing skin, unless we have removed the hair by shaving or waxing.  Hair reduces friction by rolling between the two layers of skin.  People who remove this hair often have to compensate for the loss of friction reduction by using clothing, lotions, powders, or they just deal with the occasional chafing that occurs.
  • Heat retention:  Hair makes a good insulator by trapping air close to our skin.  In order for this to work the hair has to be long enough, because of this fact the only area of the body where this works well (for most of us) is the top of the head.  Goose bumps (caused by the activation of arrector pili muscles) occur when we are cold and they have the effect of making our hair stand up more straight…  this helps to trap more air close to our skin and insulate us, this is at least somewhat effective on most of us on areas like our forearms and legs, but to be really effective the hair must long enough.
  • Gender identification:  Men and women have major differences in terms of body hair (androgenic hair) growth.  Men tend to grow much more body hair, and men grow hair on their faces, while women do not grow facial hair (at least not usually), and have less body hair.  This difference on body hair growth makes gender identification easier and more efficient and we usually help this along by identifying our with our hair style and clothing choices.
  • Communication:  There are a few ways in which we communicate with our hair.  The general appearance of a person’s hair tells you a few things about them.  If there hair is well kept, clean, and healthy appearing there is a good chance that that person is well fed, healthy, and that they generally take care of themselves.  This is useful, along with gender identification, when we are searching for potential mates.  Another way that hair is used for communication is seen in our eyebrows.  We can move our eyebrows and those movements are a major part of facial expressions.  Lastly, hair in our groin and axillary regions help to get signaling molecules called pheromones into the air.  Pheromones are scent molecules that we release, especially when sexually aroused.

Straight vs Curly vs Kinky Hair:

The amount of natural curl to a person’s hair depends on the cross sectional shape of the hair shaft.  Straight hair has a very round cross section, and the less round the hair’s cross section (the more oval or flattened) the more curly or kinky the hair, and also the lower the cross sectional area of the hair (the hair is thinner).  Hair that is very curly or kinky is also more easily broken due to its thinner nature (less cross sectional area) compared to straight hair.  Due to being oval and thus more flattened in cross section kinky hair also has a greater outer surface area to its volume, this likely leads to a more rapid loss of moisture, allowing curly and kinky hair to dry out faster.  Thus, kinky hair likely requires more moisturization.  Knowing these characteristics of different hair can be helpful in understanding how different types of hair have different care needs.

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